There are extensive published reports relating to the production of hydrophilic colloids by the aerobic propagation of bacteria of the genus Xanthomonas in aqueous nutrient media. The earliest work in this field was done at The Northern Regional Research Laboratory of the United States Department of Agriculture at Peoria, Illinois and is described in U.S. Pat. No. 3,000,790. Modified fermentation processes are described in U.S. Pat. Nos. 3,020,206; 3,391,060; 3,427,226; 3,433,708; 3,271,267; 3,251,749; 3,281,329; 3,455,786; 3,565,763; 3,594,280; and 3,391,061.
Xanthan, the exocellular anionic heteropolysaccharide produced by Xanthomonas campestris, contains mannose, glucose, glucuronic acid, O-acetyl radicals and acetal-linked pyruvic acid in the molar ratio of 2:2:1:1:0.5. This gum and its derivatives have found wide food and industrial applications. Of special interest is the increasing focus on the use of xanthan gum in displacement of oil from partially depleted reservoirs.
Typically, oil is recovered from underground reservoirs via a series of sequential operations. A new well will generally produce a limited amount of oil as a result of release of internal pressure in the well. As this pressure becomes depleted, it is necessary to pump further quantities of oil by mechanical means. These measures recover only about 25% of the total oil stored in the reservoir. A great deal of oil is still trapped within the pores of the formation. Further enhancement of recovery can then be effected by secondary recovery. In one method of recovery a waterflood is carried out by pumping water into a well or series of wells, displacing part of the trapped oil from the porous rock and collecting the displaced oil from surrounding wells. However, waterflooding still leaves about 55-60% of the available oil trapped in the formation. The explanation for this phenomenon is that water has a very low viscosity compared to the crude oil and tends to follow the path of least resistance, fingering through the oil and leaving large pockets untouched. In addition, surface forces in the formation tend to bind the oil and prevent its displacement.
A number of processes have been developed in recent years to recover further quantities of oil from these reservoirs by the use of mobility control solutions which enhance oil displacement by increasing the viscosity or permeability of the displacing fluid. Of interest are those enhanced recovery processes employing polymer flooding with a polysaccharide or polyacrylamide to increase the viscosity of the displacing fluid. Variations of this process include the use of surfactants and co-surfactants to release the oil from the rock formation. Polyacrylamides have been found to suffer such deficiencies as viscosity loss in brines and severe shear sensitivity. Since, as was well documented in the prior art, xanthan gum is insensitive to salts (does not precipitate or lose viscosity under normal conditions), is shear stable, thermostable and viscosity stable over a wide pH range, xanthan gum is a good displacing agent. Moreover, the gum is poorly adsorbed on the elements of the porous rock formations and it gives viscosities useful in enhanced oil recovery (5 to 90 centipoise units at 1.32 sec..sup.-1 shear rate) at low concentrations (100 to 3000 ppm).
The use of solutions of xanthan gum or derivatives of xanthan gum for oil recovery is described in U.S. Pat. Nos. 3,243,00; 3,198,268; 3,532,166; 3,305,016; 3,251,417; 3,319,606; 3,319,715; 3,373,810; 3,434,542 and 3,729,460. It is suggested in U.S. Pat. No. 3,305,016 that aqueous solutions containing the heteropolysaccharide in sufficient quantity to increase the viscosity be employed as the thickening agent in preparing viscous waterflooding solutions. The polysaccharide may be prepared, separated, purified and then added. Alternatively, according to this reference, the entire culture, after adding a bactericide (e.g., formaldehyde) to kill the bacteria, may be added to the flood water.